The present invention relates to an image heating apparatus suitable for an image forming apparatus such as a copying machine or a printer. In particular, it relates to an image heating apparatus which generates heat through electromagnetic induction.
For the sake of convenience, the present invention will be described with reference to an image heating apparatus (fixing apparatus) which is employed in such an image forming apparatus as a copying machine or a printer, to thermally fix a toner image to recording medium.
In an image forming apparatus, an image (toner image) is formed in an image forming station which employs a given image forming process such as an electrophotographic process, an electrostatic recording process, or a magnetic recording, is transferred onto, or directly deposited on, the recording medium (transfer sheet, electro-fax sheet, electrostatic recording sheet, OHP sheet, printing paper, formatted paper, and the like), and then is thermally fixed as a permanent image onto the surface of the recording medium by a fixing apparatus. As for such a fixing apparatus, a thermal roller type apparatus has been widely in use. However, recently, a heating apparatus which employs a film type heating system has been put to practical use, and also, a heating apparatus based on electromagnetic induction has been proposed.
FIG. 21 illustrates the essential structure of a typical electromagnetic induction based fixing apparatus in accordance with the prior technology on which the present invention is based.
A referential FIG. 10 designates a cylindrical fixing film as a rotatory member which generates heat through electromagnetic induction. The fixing film 10 comprises a heat generating layer (electrically conductive layer, magnetic layer, resistive layer) which electromagnetically generates heat.
A referential FIG. 16 designates a film guide in the form of a trough having a substantially semicircular cross section. The cylindrical fixing film 10 is loosely fitted around this film guide 16.
A referential FIG. 15 designates a means for generating a magnetic field, which is disposed on the inward side of the film guide 16, and is constituted of an excitation coil 18 and a magnetic core 17.
A referential FIG. 30 designates an elastic pressure roller, which is disposed so that it presses, with a predetermined pressure, upon the bottom surface of the film guide 16, with the fixing film interposed, and forms a fixing nip N having a predetermined width. The magnetic core 17 of the magnetic field generating means 15 is squarely aligned with the fixing nip N.
The pressure roller 30 is rotatively driven in the counterclockwise direction, indicated by an arrow mark, by a driving means M. As the pressure roller 30 is rotatively driven, the fixing film 10 is driven in the clockwise direction indicated by another arrow mark, by the friction between the pressure roller 30 and the outward surface of the fixing film 10, with the inward surface of the fixing film 10 sliding flatly on the bottom surface of the film guide 16; the fixing film 10 is rotated along the outward surface of the film guide 16 at a peripheral velocity substantially equal to the peripheral velocity of the pressure roller 30 (pressure roller driving system).
The film guide 16 plays a role in generating pressure in the fixing nip N, supporting the excitation coil 18 and magnetic core 17 of the magnetic field generating means 15, supporting the fixing film 10, and stabilizing the conveyance of the fixing film 10 while the fixing film 10 is rotatively driven. The film guide 16 is formed of dielectric material which does not interfere with the permeation of magnetic flux, and also is capable of withstanding the load it must bear.
The excitation coil 18 generates an alternating magnetic flux as it is supplied with an alternating electric current by an unillustrated excitation circuit. Since the alternating magnetic flux is generated so as to be concentrated to the fixing nip N, the heat generated through electromagnetic induction is also concentrated to the fixing nip N. In other words, the fixing nip N is very efficiently heated.
The temperature of the fixing nip N is controlled by a temperature controlling system inclusive of a temperature detecting means; it is maintained at a predetermined level by controlling the current supplied to the excitation coil 18.
Reviewing the above description, as the pressure roller 30 is rotatively driven, the cylindrical fixing film 10 is rotated around the film guide 16, and electrical current is supplied to the excitation coil 18 from the excitation circuit to generate heat in the fixing film 10 through electromagnetic induction. As a result, the temperature of the fixing nip N is increased. As the temperature of the fixing nip N reaches the predetermined level, it is maintained at this level. With the heating apparatus in this state, a recording medium P, on which a toner image t has been just deposited without being fixed thereto, is introduced into the fixing nip N, between the fixing film 10 and the pressure roller 30, with the image bearing surface of the recording medium P facing upward so that it will come in contact with the outward surface of the film 10. Then, the recording medium P is passed through the fixing nip N, along with the fixing film 10, while being compressed by the pressure roller 30 and the film guide 16, with the image bearing surface being flatly in contact with the outward surface of the fixing film 10. While the recording medium P with the toner image t is passed through the fixing nip N as described above, the toner image t which is borne on the recording medium P, but is yet to be fixed, is heated by the heat electromagnetically induced in the fixing film 10, being thereby fixed to the recording medium P. After passing through the fixing nip N, the recording medium P separates from the outward surface of the rotating fixing film 10, and is conveyed further to be discharged from the image forming apparatus.
In terms of preciseness in heating a toner image using a fixing apparatus which employs an electromagnetic induction system such as the system described above, it is desirable that the temperature detecting means of the fixing apparatus detects the temperature of the fixing film 10 itself, which actually comes in contact with the toner image t. However, if a temperature detection element for measuring the temperature of the fixing film 10 is placed in contact with the outward surface of the fixing film 10, the film surface is liable to be damaged, and if the film surface is damaged, the damaged surface causes the offset of the fixed toner image. This is one of the problems of the image heating apparatus based on the prior art. In addition, if the fixing film 10 is rotated at an extremely high speed, it is rather difficult to maintain stable contact between the temperature detection element and the fixing film 10, hence the accuracy of the detected temperature deteriorates. As a result, the temperature of the fixing film 10 cannot be reliably controlled, which is another problem.
The object of the present invention is to provide an image heating apparatus capable of detecting the temperature of a moving member without damaging the surface of the moving member which generates heat through electromagnetic induction.
Another object of the present invention is to provide an image heating apparatus in which stable contact is maintained between a moving member which generates heat through electromagnetic induction, and a temperature detecting means.
Another object of the present invention is to provide an image heating apparatus in which a temperature detecting means is in contact with the inward facing surface of an endless moving member which generates heat through electromagnetic induction.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.